1. Field of the Invention
The present invention relates to a cylinder apparatus and particularly to a cylinder apparatus in which a piston is movably inserted into a cylinder bore via a cup seal.
2. Description of Related Art
In a cylinder apparatus, it is known that an annular seal groove is formed on an inner circumferential surface of a cylinder bore into which a piston is movably inserted, and a cup seal for slidably sealing the portion between the cylinder bore and the piston is fitted into the seal groove.
In general, as described in Japanese Patent Unexamined Publication JP-A-2004-231093, the seal groove includes: a seal groove bottom surface extending in a circumferential direction; a bottom-side-surface on a cylinder bore bottom portion side; an opening-side-surface on a cylinder bore opening portion side; and a seal groove opening opened to a piston. Further, the cup seal includes: a base portion disposed on the cylinder bore opening portion side of the seal groove, an inner circumferential lip portion extended from an inner circumferential side of the base portion toward the cylinder bore bottom portion and having an inner circumferential surface which slides relative to an outer circumferential surface of the piston; and an outer circumferential lip portion extended from an outer circumferential side of the base portion also toward the cylinder bore bottom portion and having an outer circumferential surface which abuts against the seal groove bottom surface.
In addition, for improving sealing property of the cup seal, an annular protruding portion protruding toward the piston side is formed on the inner circumference of the base portion. However, when such an annular protruding portion is formed, there are cases where the annular protruding portion is bitten into a passage hole provided in the piston when the operation of the piston is started. In order to prevent this situation, as described in Japanese Patent Unexamined Publication JP-A-2003-194100, it is proposed to provide a tapered surface on the inner circumference of the base portion which becomes wider toward the outer diameter side.
In the cup seal described in JP-A-2004-231093, in a process where the piston moves forward toward the cylinder bore bottom portion to pressurize a hydraulic fluid in a hydraulic pressure chamber during the operation, the base end surface of the base portion of the cup seal is pressed against an opening-side-surface of the seal groove. At this time, when the contact pressure between the base end surface and the opening-side-surface is increased, the base portion is made less likely to extend in a radial direction of the cylinder, and a sealing force generated by pressing the inner circumferential lip portion positioned on the piston side against the outer circumferential surface of the piston is made less likely to increase. As a result, there is a possibility that an ineffective stroke becomes unstable.
Further, in the cup seal described in JP-A-2003-194100, there is provided a tapered surface which becomes wider toward the outer diameter side and intersects the radial direction of the cylinder bore at an obtuse angle on the inner circumference of the base portion, and the tapered surface is not brought into pressure contact with the piston even when the hydraulic fluid in the hydraulic pressure chamber is pressurized. As a result, the sealing force at the base portion of the cup seal is made less likely to increase similarly to the above-described case, and there is a possibility that the ineffective stroke becomes unstable also similarly to the above-described case.